Structural dynamics and phase transitions


Phase change materials: Structural dynamics and optical properties in a resonantly bonded material

A limiting case of non-equilibrium electron-lattice interaction is the situation of electronic excitation inducing a structural phase transition. Such phase transitions may occur in a thermal fashion, i.e. after thermal equilibration between electrons and lattice, or nonthermally as a direct consequence of electronic excitation-induced changes to the lattice potential. The latter scenario occurs predominantly in covalently bound crystals as optical excitation transfers population from bonding to antibonding states.

Phase change materials (PCMs) exhibit two quasi-stable states with large contrast in optical, electrical and crystalline properties, which may be repeatedly switched by appropriate heating and cooling cycles employing either light or electrical current. This class of materials is widely used as rewritable data storage. In collaboration with the research group of Simon Wall (ICFO Barcelona), we investigate the nature of ultrafast phase transitions of the phase change material Ge2Sb2Te5 (GST). The ultrafast optical and structural response of GST to femtosecond photoexcitation is investigated by optical spectroscopy and femtosecond electron diffraction. We perform time-resolved experiments employing both weak excitation below the threshold for light-induced phase transitions as well as single-shot excitation inducing a persistent crystalline-to-amorphous phase transition. The time-dependent dielectric function is obtained by simultaneous measurement of transient absorption and reflectivity. The timescale of heating of the lattice by energy transfer from electrons to phonons as well as the time of the phase transition is revealed by time-resolved diffraction.

We observe that the optical and structural properties respond on different time scales to the electronic excitation. While the dielectric function changes virtually immediately, the response of the lattice is governed by the rate of energy transfer to lattice vibrations. We interpret this observation in terms of depletion of resonantly bonded electrons, a fraction of the valence electrons with large contribution to the optical properties of the crystalline state but less important for the stability of the crystalline structure.

Further reading:

L. Waldecker, T. A. Miller, M. Rude, R. Bertoni, J. Osmond, V. Pruneri, R. Simpson, R. Ernstorfer, and S. Wall:
Time-domain separation of optical properties from structural transitions in resonantly bonded materials.
Nature Materials AOP (2015), [doi: 10.1038/nmat4359]

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Ultrafast Evolution of the Excited-State Potential Energy Surface of TiO2 Single Crystals Induced by Carrier Cooling

Transition metal oxides like TiO­2 exhibit a pronounced coupling of electronic and lattice degrees of freedom. As valence and conduction bands are formed from different atomic orbitals, namely O 2p and Ti 3d, respectively, interband excitation effectively causes a charge re-distribution within the unit cell modifying the potential energy surface of the lattice. We investigated the effect of electron temperature and carrier relaxation on the excited-state potential energy surface prepared by a few-fs ultraviolet laser pulse. The analysis of the phase of the induced coherent Ti-O stretch vibration reveals a dynamic evolution of the lattice potential synced to the extremely fast cooling of charge carriers. This correlation between electron temperature and the lattice potential is in agreement with non-equilibrium density functional theory calculations performed by Eeuwe Zijlstra and Martin Garcia, Universität Kassel.

TiO2

Illustration of the effect of carrier excitation and relaxation on the lattice potential energy surface of titanium dioxide. An extremely short ultraviolet pulse creates hot excited electrons in the semiconductor titanium dioxide. This changes the spatial distribution of the electrons within the lattice, resulting in a shift of the potentials for the atomic cores, i.e., their rest position (central picture). The subsequent cooling of the electrons, which takes about 20 femtoseconds, further amplifies this effect (right picture). The combined effect of electron excitation and cooling leads to a force on the oxygen atomic cores, resulting in a coherent oscillation within the crystal structure.

Further reading:

  • E.M. Bothschafter, A. Paarmann, E.S. Zijlstra, N. Karpowicz, M.E. Garcia, R. Kienberger, and R. Ernstorfer:
    Ultrafast Evolution of the Excited-State Potential Energy Surface of TiO2 Single Crystals Induced by Carrier Cooling.
    Phys. Rev. Lett. 110, 067402 (2013), [doi: 10.1103/PhysRevLett.110.067402].

Electron-lattice-spin interaction in 2D semiconductors: transition metal dichalcogenides

Two-dimensional crystalline materials offer fascinating properties rendering possible new concepts for optoelectronic, spintronic or valleytronic devices. In addition to semi-metallic graphene, research on transition-metal dichalcogenides (TMDC) has revived as several of these compounds are semiconductors with a unique set of properties. All TMDCs comprise of covalently bound single-crystalline layers of transition metals atoms sandwiched by two planes of group VI elements and van der Waals interlayer interaction. In the limit of monolayer crystals, the TMDCs consisting of Mo or W and S, Se or Te are direct-band gap semiconductors showing large excitonic effects with excitons stable at room temperature. The strong spin-orbit coupling and the lack of in-plane inversion symmetry of these materials additionally result in unusual electron-spin correlations, in particular at the K points in reciprocal space, where the top of the valence band exhibits spin-split bands separated by several 100 meV and with alternating spin polarization between neighboring valleys.

We investigate this wealth of coupling and correlation effects in these materials by femtosecond electron diffraction, and time-resolved optical and photoelectron spectroscopy.

Electron-phonon interaction in quantum-confined systems

Under conditions of strong spatial confinement in all dimensions the translational symmetry is strongly violated and Bloch states, for both electrons and phonons, transform into discrete molecular-like states. If the spacing between the levels is larger than the thermal broadening (kT) then the system will essentially resemble a ‘superatom’ or a ‘giant molecule’. Such nearly zero-dimensional systems are known as nanoclusters. Nanoclusters have interesting and often peculiar properties even above the quantum-confinement limit because of their tremendous surface-to-volume ratio. Ongoing research in our team, in collaboration with the team of Professor Richard Palmer in Birmingham University, aims to ellucidate how the electrons interact with the lattice in such quantum-confined systems and in addition what is the role of their chemical environment with which they can exchange electronic and vibrational heat.

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